The direct upconversion (or zero-IF, where IF is the acronym for intermediate frequency) transmitter is the transmitter architecture typically used in wireless transceivers. In such a transmitter, two mixers are driven by baseband (BB) and local oscillator (LO) signals, which are in quadrature. Ideally, combining the two signal paths as BBI·LOI−BBQ·LOQ provides a single output frequency ωLO+ωBB. However, there are analog imperfections such as direct current (DC) offsets (δBBi,q, δLOi,q), LO-to-RF (where RF is the acronym for radio frequency) feedthrough (σLOi, σLOq), and quadrature amplitude (εBB, εLO) and phase (ΔφBB, ΔφLO) errors. These imperfections cause an output spectrum containing an image and a carrier signal. High-end telecom systems, such as a wireless local area network (WLAN), require suppression of these spurs. In order to determine the exact origin and contribution of the analog non-idealities to this output spectrum, the amplitude and phase of each spur must be known. In the prior art, this is done by downconverting the RF spectrum back to baseband with a substantially ideal receiver, i.e., an expensive device that has a substantially higher conversion accuracy than the transmitter, which is necessary since otherwise the measurement will be as erroneous as the errors that are to be determined. According, this method is not viable for automatic calibration.
Furthermore, up to now only techniques using the amplitude information are known. This amplitude information is obtained by placing a peak detector at the RF output [M. Faulkner, T. Mattsson, and W. Yates, “Automatic adjustment of quadrature modulators”, IEE Electronic Letters, vol. 27, no. 3, pp. 214-215, January 1991] or by monitoring the power in an adjacent channel [D. Hilborn, S. Stapleton, and J. Cavers, “An adaptive direct conversion transmitter”, IEEE Trans. On Vehicular Technology, vol. 43, no. 2, pp. 223-233, May 1994]. As no phase information on the spurs is available, a time-consuming recursive “trial-and-error” algorithm is needed in order to determine the optimal baseband corrections that result in the smallest error.
Thus, there is a need for a direct upconversion transmitter that can be calibrated without needing an expensive ideal receiver and for a method in which the device characteristics or non-idealities can be determined and compensated in a simpler way, avoiding a time-consuming recursive algorithm. Since the non-idealities of a direct upconversion transmitter largely originate from frequency conversion circuitry used and the frequency conversion circuitry is also used in other electrical devices, there is a more general need to provide any electrical device that comprises frequency conversion circuitry with a means for more simply and less expensively deriving the device characteristics, and to provide a simpler method for compensating the characteristics of such a device.
It is therefore an aim of the invention to provide an electrical device, a method for determining device characteristics, and a method for compensating the characteristics which fulfill the above mentioned needs.